How to appeal a health insurance denial: a step-by-step guide

Getting a denial letter from your health insurer feels final. It is not. A denial is the opening move in a process that you have a legal right to challenge, and insurers reverse their own decisions far more often than most people expect. The hard part is that the rules, deadlines, and paperwork are designed to be confusing, and many patients give up before they ever file. This guide walks through how to appeal a health insurance denial from the moment the letter lands in your mailbox to the independent review that can overturn the insurer for good.

Whether your claim was rejected as "not medically necessary," for a missing prior authorization, or for an out-of-network technicality, the same core process applies. Work through it in order, keep copies of everything, and do not let a deadline pass.

Step 1: Read your denial letter and find two critical facts

Before you do anything else, read the denial letter (sometimes called an Explanation of Benefits, or EOB, or an adverse benefit determination) and pull out two things:

• The denial reason and code. Insurers use standardized claim adjustment reason codes. For example, CO-50 means "not medically necessary," CO-197 points to a missing prior authorization, and PR codes assign cost to the patient. The code tells you precisely what you have to rebut. If you are unsure what yours means, look it up in our denial code library.
• The appeal deadline. Every denial letter must tell you how to appeal and by when. This is the single most important date in the entire process. Write it on your calendar the day the letter arrives.

If the letter is vague about why the claim was denied, that itself is a problem you can raise on appeal. The law requires insurers to give you a specific reason and to point to the plan provision or clinical standard they relied on.

Step 2: Request your plan documents and claim file

You cannot effectively rebut a denial you do not fully understand. Before you write a word of your appeal, request the records the insurer used to make its decision. For employer-sponsored plans governed by ERISA, the federal claims-procedure regulation at 29 CFR § 2560.503-1(m)(8) entitles you to copies of all documents relevant to your claim, free of charge. That includes:

• The Summary Plan Description and the full plan document or policy.
• The internal medical-necessity criteria and clinical guidelines the plan applied to your case.
• The identity and qualifications of the medical reviewer who signed off on the denial.
• Any expert reports or notes generated during the review.

Send this request in writing and keep a copy. Once you have the plan's own criteria in hand, you can show, point by point, why your care actually meets them. This is also where you gather the clinical ammunition: your physician's notes, test results, imaging, and a letter of medical necessity. For more on ERISA-specific rights and the broader framework, see our guide to ERISA appeals.

Step 3: File your internal appeal

The internal appeal is your formal written request for the insurer to reconsider. This is the stage where most cases are won or lost, so treat it as the main event, not a formality. A strong denied claim appeal letter does five things:

1. Identifies the claim precisely. Include the member ID, claim number, date of service, provider, and the exact denial reason and code you are appealing.
2. States plainly that you are appealing. Use the words "I am appealing this denial" so there is no ambiguity about what you are requesting.
3. Rebuts the specific reason for denial. If the insurer said the care was not medically necessary, quote the plan's own medical-necessity definition and show how your situation satisfies it, backed by your doctor's records and relevant clinical evidence.
4. Attaches the supporting documents. A letter of medical necessity from your treating physician, clinical notes, peer-reviewed evidence, and any relevant guidelines.
5. Cites the law where it helps. Reference the protections that apply to your plan, such as the ERISA claims procedure under 29 USC § 1133 or the ACA internal-appeal rights under ACA section 2719.

Send the appeal by certified mail with return receipt, or through the insurer's official portal with a confirmation you save. You want indisputable proof of what you sent and when. If your insurance comes from a private employer, front-load everything: under ERISA, the record you build during the internal appeal is often the only evidence a court will later consider, so do not hold anything back for "later."

If you would rather not draft this from scratch, our free appeal letter generator reads your denial and produces a structured, cited appeal you can edit and send.

Step 4: Know your deadlines and the insurer's deadlines

Deadlines run in both directions, and both matter. For your side, employer plans governed by ERISA must give you at least 180 days from receipt of the denial to file an internal appeal, under 29 CFR § 2560.503-1(h)(3)(i). A plan may grant more time but never less. Marketplace and other ACA-regulated plans follow comparable minimums.

The insurer is on a clock too. Under 29 CFR § 2560.503-1 (and, for ACA-regulated plans, 45 CFR § 147.136), the plan must generally decide:

• Pre-service appeals within 30 days (care you have not yet received).
• Post-service appeals within 60 days (care already provided and billed).
• Urgent appeals within 72 hours (see Step 7 below).

If the insurer misses its own deadline, you may be treated as having exhausted the internal process, which lets you move straight to external review. Track every date in writing. For a deeper breakdown of timelines by plan type, our blog covers how long you have to appeal.

Step 5: Request an external review

If the internal appeal is denied, you are not out of options. The Affordable Care Act, codified at 42 USC § 300gg-19 and implemented at 45 CFR § 147.136, requires non-grandfathered health plans to offer an external review after you have exhausted internal appeals. This is the part of the process patients most often skip, and it is one of the most powerful.

In an external review, an independent medical expert with no financial stake in your plan re-examines the denial. Crucially, the reviewer's decision is binding on the insurer: if the independent reviewer says the care should be covered, the plan must cover it. Because the reviewer is genuinely neutral, external review overturns a meaningful share of denials that the insurer upheld internally.

You generally have four months from the final internal denial to request external review, and the request is often a short standard form. To understand how independent review organizations work and how to prepare your submission, read our guide to external review and independent review organizations.

Step 6: File a state insurance commissioner complaint as a parallel track

Your formal appeal is not the only lever. Most states let you file a complaint with the state insurance commissioner or department of insurance. These regulators oversee fully-insured plans and can investigate unfair claims handling, prompt-payment violations, and patterns of bad-faith denials.

A complaint does not replace your internal appeal or external review, and it does not pause your deadlines, so keep your formal appeal moving. But it can add real pressure, prompt a second look, and document a pattern of misconduct. One caveat: fully self-funded employer plans are governed by ERISA and generally fall outside state insurance regulation, so the commissioner route is strongest for fully-insured and individual-market plans. Run both tracks in parallel where you can.

Step 7: Use an expedited appeal when care is urgent

If waiting weeks for a decision could seriously jeopardize your life, your health, or your ability to regain maximum function, you do not have to wait. Request an expedited (urgent) appeal. Under 29 CFR § 2560.503-1 and 45 CFR § 147.136, urgent appeals must be decided within 72 hours.

A few things make urgent appeals different:

• You can usually start them by phone rather than waiting on certified mail, then follow up the same day in writing.
• You can request an expedited external review at the same time as the internal appeal, rather than waiting for the internal process to finish.
• A short note from your physician explaining why the situation is time-sensitive carries a lot of weight.

If your care is genuinely urgent, say so explicitly and in writing. The 72-hour clock only starts when you ask for the expedited track.

Putting it together

The pattern behind every successful appeal is the same: figure out the exact reason your insurance claim was denied, gather the plan's own rules and your clinical evidence, and then rebut the denial directly, on time, in writing, with proof of delivery. File the internal appeal first, escalate to external review if needed, and run a state complaint in parallel. The patients who win are rarely the ones with the strongest cases; they are the ones who actually file and meet the deadlines.

If the paperwork feels overwhelming, you do not have to do it alone. Counterclaim turns your denial letter into a structured, cited denied claim appeal letter in minutes, free, so you can spend your energy on the evidence rather than the formatting.